1. Field of the Invention
This invention relates to a gallium alloy for dental restorations, and particularly to a powder-liquid mixture containing a gallium alloy which is hardenable after triturating a liquid gallium alloy with a metal powder, especially a silver base alloy powder, applied for the purpose of replacing dental amalgam containing mercury.
2. Discussion of the Background
A dental amalgam as a hardenable mixture is prepared by mixing liquid mercury with a silver alloy powder. This amalgam is filled into a tooth cavity, and hardened at mouth temperature by an amalgam reaction between mercury and silver or other components yielding strength durable to occlusal pressure. Since the amalgam has good workability in preparation, it has long been used up to the present.
However, as to mercury the toxicity of its vapor has been formerly known, and environmental pollution caused by mercury has recently become a social problem in many countries.
Suggestions of using gallium in place of mercury date as far back as 1928 and subsequent attempts have been made, because gallium has a melting point (mp. 29.78.degree. C.) next lower to that of mercury (m.p. -38.86.degree. C.).
From a metallurgical viewpoint, gallium constitutional diagrams are known for binary systems such as Ga-Zn (eutectic temperature 25.degree. C.), Ga-Sn (eutectic temp. 20.degree. C.) and Ga-In (eutectic temp. 15.7.degree. C.). However, gallium ternary systems have been scarcely known.
Most research concerning non-mercury metal filling materials for dental application has been made using gallium metal or gallium binary eutectic alloys known as Ga-Sn, Ga-Zn and Ga-In together with powder such as palladium, silver, gold, copper, tin, zinc and silver-tin-copper alloy. Of these, J. P. Lyle et al. disclosed in U.S. Pat. No. 2,585,393 (1952) an alloy composed of 49-74% nickel, 25-45% gallium and 0.5-7.5% silicon as a metallic composition, prepared by admixing a powder mixture of nickel and silicon with a proper portion of gallium by warming it slightly to form a liquid.
D. L. Smith et al. reported a compound for filling teeth cavities containing 27-31% gallium, 29-30% tin and 40-43% copper, by preparing Ga-Sn eutectic alloy with powdered copper-tin alloy (Cu.sub.3 Sn) in The Journal of the American Dental Association, Vol. 53, P 677 (1956) and also disclosed an alloy consisting of 20-40% of liquid gallium and 60-80% powdered cobalt in U.S. Pat. No. 2,864,695 (1958).
S. T. Rogova et al. disclosed similar alloy of Ga-Sn-Cu system in U.S. Pat. No. 4,015,981 (1977).
R. M. Waterstrat reported gallium-palladium-tin alloy for restorative dentistry in The Journal of the American Dental Association, Vol. 78, P 536 (1969).
In Japanese Pat. No. 1,059,723 (Japanese Patent Publication No. 48091/1980), T. Yoshida claimed a method of preparation of metal filling materials for dental restorations which comprises adding 1-13.5% tin and less than 24.5% indium or less than 5% zinc to gallium so as to lower the melting point of gallium temporarily when triturating them with metal powder. Furthermore, Japanese Pat. No. 1,075,467 (Japanese Patent Publication No. 15453/1981) disclosed alloy powder to be mixed with a binary alloy consisting of gallium and 1-13.5% tin. In these patents, examples are shown relating to the combinations of Ga-Sn binary alloys with single metal powder such as Ag, Au, Cu and Sn or alloy powder such as Cu.sub.3 Sn, Ag-Cu-Sn and that consisting of 85-55% silver and 15-45% copper to which one or more of (i) less than 15% gold, (ii) less than 15% palladium and (iii) less than 30% tin are further added. However, no examples concerning filling material or hardened product comprising an alloy powder with a ternary or higher liquid gallium alloy having a melting point lower than the eutectic temperature (15.7.degree. C.) of Ga-In alloys were presented in these patents.
The fact that the filling materials containing gallium for dental restorations have not been commercialized up to the present is due to the facts that the melting point of gallium alloys could not be lowered to a practical level, that gallium alloys are difficult in handling and workability as compared with conventional amalgams, and that various properties are not satisfactory for dental restorative materials. In view of the facts, the applicant has engaged in research and development concerning multicomponent liquid gallium alloys as described in Japanese Patent Application Nos. 58-240933 to 58-240936, and others.